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Light harvesting in natural photosynthesis is accomplished by pigment–protein complexes known as antenna complexes. These are associated with the photosystems, and their role is to collect as much as possible of the available light and funnel the excitation energy to them. The photosynthetic pigments are mostly bacteriochlorophylls, chlorophylls, and carotenoids (carotenes and xanthophylls). Figure 3.3 shows some of the chlorophylls utilized by photosynthetic organisms. They differ in substituents on ring A at positions 2 and 3 and on ring B at position 7 of the chlorin system. Chlorin differs from the porphin tetrapyrrole in having a reduced D ring, while in bacteriochlorins the B ring is also reduced. The principal characteristic of the electronic structure of chlorophyll‐type pigments is the conjugated π orbital system that is delocalized over the aromatic rings. Differences in substituents such as those shown in Figure 3.3 create slight variations in electronic structure that contribute to distinct optical properties by affecting the nature and energies of π–π* excitations, the lowest of which falls within the red or near‐infrared (NIR) region. Thus, among the four types of chlorophyll shown in Figure 3.3, with respect to the most common chlorophyll a (Chl a) that has an absorption maximum (λ_max) at ca. 662 nm in vitro, Chl b is blueshifted (λ_max = 644 nm) and Chl d is redshifted (λ_max = 697 nm) [4]. Chl f was discovered only recently [21] and is the most redshifted chlorophyll known in oxygenic photosynthesis (λ_max = 707 nm). It has been suggested that its presence in key positions within PS‐I and PS‐II enables charge separation to occur with far‐red light [22].

Figure 3.3 Selected types of chlorophyll molecules encountered in natural photosynthesis.

The absorption profile of photosynthetic pigments is a crucial determinant of the ability of organisms to utilize the wavelengths of sunlight available within their particular ecological environment. Besides the availability of distinct molecular species, fine‐tuning the optical properties of individual (bacterio)chlorophylls can be achieved by (i) variable axial ligation at the Mg center, (ii) out‐of‐plane distortions of the (bacterio)chlorin ring, and (iii) the structured electrostatic environment of the protein matrix. These effects refer to the role of the protein in optimizing locally the properties of individual pigments. However, the truly amazing role of the protein in antenna complexes operates at the much larger scale of the precise and stable arrangement of arrays of pigment molecules (tens to hundreds of them) in terms of relative orientations and distances and in the control of their optical properties at the system level. The protein scaffold thus controls and optimizes the overall light‐harvesting profile of the antenna complex, maximizes the effective pigment concentration while avoiding concentration quenching, and ensures efficient and directional energy transfer to the charge separation site. At the same time, antenna complexes incorporate mechanisms of photoprotection, which become particularly important under conditions of high light intensity. Photoprotection refers principally to eliminating chlorophyll triplet states that can persist long enough to react with molecular oxygen and is typically achieved by incorporating carotenoids within the protein matrix. These can play a dual role as light harvesters at wavelengths not covered by chlorophylls, i.e. green and blue light, and as chlorophyll triplet‐state quenchers. There is a great diversity of antenna complexes in biology [23], characterized by variability in pigment composition and organization as well as in the mode of association with their respective reaction center complexes. Figure 3.4 depicts selected examples of light‐harvesting complexes ranging from bacteria to higher plants.

Figure 3.4 (a) Side and top view of the light‐harvesting complex LH2 from the purple bacterium R. acidophila 10050 (PDB: 1KZU [24]), showing bacteriochlorophyll a pigments in green and carotenoids (rhodopin glucoside) in orange. (b) Rod structure of c‐phycocyanin from the phycobilisome light‐harvesting antenna of cyanobacterium T. vulcanus (PDB: 3O18 [25]). (c) Light‐harvesting complex II (LHCII) from pea (PDB: 2BHW [26]), showing Chl a pigments in dark green and Chl b in light green.

Efficient unidirectional singlet energy transfer among chromophores relies on adequate spectral overlap between the emission of the donor pigment and the absorption of the acceptor pigment and on successful avoidance of alternative excited‐state decay pathways. Suitable energy gradients that involve sacrificing some of the excitation energy ensure rapid transfer along the productive direction. The principal theory of excitation energy transfer is that of Förster resonance energy transfer (FRET) [27]. This relates the rates of excitation energy transfer to the dipole–dipole interactions between the donor and acceptor chromophores. The transfer rate according to FRET is determined by the relative orientations of the local electronic transition dipoles and the distance between them, and the theory formally applies to pigments that are spatially well separated, or equivalently when the electronic coupling between pigments is negligible. These conditions are rarely fully met in actual biological antenna complexes; therefore FRET most often provides a rough approximation at best. In this case a more appropriate approach is based on extensions of the Redfield theory [28] that treat the strong excitonic coupling non‐perturbatively and take into account interactions with the environment. The close proximity of chromophores within the protein matrices implies strong electronic interaction between donor and acceptor pigments that results in shared excitonic energy levels that lie lower than those of the independent pigments. It is currently accepted that quantum coherence plays a key role in the remarkable efficiency of light harvesting in natural photosynthesis [29–31].

Considering biomimetic approaches to artificial versions of antenna complexes, certain challenges become immediately apparent. The pigments used in biological light harvesting are rather small molecules that are elaborately positioned and electronically fine‐tuned by a “smart” protein matrix, resorting only to limited extent to covalent bonding. In this respect, it is tempting to consider the possible role of template‐guided assembly of pigments as opposed to the conventional synthetic approach of covalently linking arrays of chromophores. It is remarkable that even when using several molecules of the same pigment, the protein matrix can modulate their absorption profile to produce a range of site energies with well‐defined spatial distribution. This is important for expanding the spectral range and light‐harvesting ability of the antenna beyond the intrinsic features of a given pigment, but its directed nature makes it also the basis of a crucial functionality: the creation of energy gradients within the antennae that enable efficient transfer of excitation energy to the reaction centers. It is likely that similar functionality could be built synthetically by utilizing ordering of distinct chromophores rather than manipulating the properties of a given pigment in a site‐dependent manner. The high effective concentration of chromophores achieved in antenna complexes also appears hard to achieve in artificial analogs while avoiding concentration quenching (self‐absorption) and seems to be possible in nature only because of the pigment organization imposed by the protein scaffold. Finally, the adaptability to changing light conditions, for example, by rerouting excitation energy transfer pathways, and the intrinsic photoprotection mechanisms of photosynthetic enzymes and antenna complexes are features that would be difficult, though not impossible [32], to replicate outside biology.

Artificial light‐harvesting complexes [33, 34] can be conceived in the context of supramolecular chemistry as arrays of chromophores coupled through covalent linkages based on dendrimer architectures or assembled on scaffolds. Dendrimers [35, 36] lend themselves to V‐shaped or circular arrangements of covalently bonded chromophores and can exhibit inherent directionality in excitation energy transfer at the molecular level. An important design concept in this area is the linkage of different but complementary chromophores to enable wide spectral coverage and to create energy gradients for efficient excitation energy transfer cascades. An additional requirement for a synthetic antenna complex is the successful interfacing with the synthetic reaction center. The coupling of the two modules should ensure efficient energy transfer from the light‐harvesting system to the charge separation site while avoiding uncontrolled perturbation of the latter by the former.

Dendrimeric synthetic antenna complexes have been constructed using transition metal complexes of Ru or Os, bridged by oligopyridine‐type ligands [37]. The choice of connecting ligand is important because it does not merely bring together mononuclear complexes into a closely packed ensemble, but it determines the overall nanoscale architecture of the dendrimeric structure and controls both the local electronic properties of the metal‐based units and the electronic coupling between them [34]. Earth‐abundant first‐row transition metal complexes feature much less prominently in the field of light harvesting compared to 4d and 5d elements, but research efforts are currently directed toward changing this paradigm [38]. Multiporphyrin arrays, particularly utilizing zinc porphyrins, represent another common pattern in the context of bioinspired light‐harvesting complexes [33, 34]. On the other hand, light‐harvesting dendrimers constructed using solely organic subunits have also been explored [42]. Figure 3.5 depicts two examples representative of ideas mentioned above. Another molecular approach worth mentioning is based on host–guest constructions, where molecules such as organic dyes or transition metal complexes are accommodated within internal cavities of dendrimers [45, 46]. Research and development of artificial light‐harvesting systems is an active field with immense potential and diversity that goes far beyond what can be covered in the present chapter, so the interested reader is referred to the primary literature for more details.

Figure 3.5 Examples of synthetic approaches to multi‐chromophore arrays: (a) a nine‐porphyrin array unit comprising a central free‐base porphyrin core that acts as final acceptor and is surrounded by eight energy‐donating zinc porphyrins [43].

Source: Choi et al. [41]. (b) Dendrimer consisting of a terrylenediimide (TDI) core with four attached perylenemonoimides (PMI) and eight peripheral naphthalenemonoimides (NMI) [44].

Source: Balzani et al. [34].